Oligodendrocyte responses to buprenorphine uncover novel and opposing roles of μ‐opioid‐ and nociceptin/orphanin FQ receptors in cell development: Implications for drug addiction treatment during pregnancy

Abstract

While the classical function of myelin is the facilitation of saltatory conduction, this membrane and the oligodendrocytes, the cells that make myelin in the central nervous system (CNS), are now recognized as important regulators of plasticity and remodeling in the developing brain. As such, oligodendrocyte maturation and myelination are among the most vulnerable processes along CNS development. We have shown previously that rat brain myelination is significantly altered by buprenorphine, an opioid analogue currently used in clinical trials for managing pregnant opioid addicts. Perinatal exposure to low levels of this drug induced accelerated and increased expression of myelin basic proteins (MBPs), cellular and myelin components that are markers of mature oligodendrocytes. In contrast, supra-therapeutic drug doses delayed MBP brain expression and resulted in a decreased number of myelinated axons. We have now found that this biphasic-dose response to buprenorphine can be attributed to the participation of both the μ-opioid receptor (MOR) and the nociceptin/orphanin FQ receptor (NOP receptor) in the oligodendrocytes. This is particularly intriguing because the NOP receptor/nociceptin system has been primarily linked to behavior and pain regulation, but a role in CNS development or myelination has not been described before. Our findings suggest that balance between signaling mediated by (a) MOR activation and (b) a novel, yet unidentified pathway that includes the NOP receptor, plays a crucial role in the timing of oligodendrocyte maturation and myelin synthesis. Moreover, exposure to opioids could disrupt the normal interplay between these two systems altering the developmental pattern of brain myelination.

The Opioid System and Brain Development: Effects of Methadone on the Oligodendrocyte Lineage and the Early Stages of Myelination

“…Oligodendrocytes were isolated from either 3-day-old (oligodendrocyte progenitors) or 9-day-old (pre-oligodendrocytes) Sprague-Dawley rat brains using a Percoll gradient (GE Healthcare Bio-Sciences, Pittsburgh, PA) and differential adhesion as previously reported [22,34]. Briefly, brain meninges and main blood vessels were removed by gently rolling the tissue on sterile filter paper.…”

“…All electrophoresis reagents and supplies were from Bio-Rad Laboratories (Hercules, CA). The membranes were then subjected to western blot analysis as previously reported [22], using the following primary antibodies: anti-MBP (Millipore, Temecula, CA, raised against 82-87 peptide, dil.1:100), anti-MOG (Millipore, dil. 1:2,000), anti-PLP (Abcam Inc., Cambridge, MA, dil.…”

“…with 4% paraformaldehyde in PBS and double immunocytochemical staining was carried out as reported before [22], with minor modifications. Non-specific antibody binding was blocked by incubation for 1 hour in PBS with 0.5% normal goat serum and 0.05% Tween-20 (blocking solution).…”

“…Importantly, there is increasing evidence to support a crucial function of the endogenous opioid system in myelination. Neural stem cells and cells at different maturational stages along the oligodendroglial lineage express opioid receptors, and their modulation in culture has been shown to induce both mitogenic and differentiating effects [19-22]. These findings raise the possibility that interference with the endogenous opioid system by drug abuse and maintenance treatments for opioid addicts may have an effect on brain myelination.…”

“…While supra-therapeutic doses of this drug delayed myelination, therapeutic doses resulted in accelerated and increased brain expression of myelin basic proteins (MBPs), markers of mature myelin and differentiated oligodendrocytes [31]. The finding of a bell-shaped dose-response led us to the discovery of a novel mechanism that controls oligodendrocyte maturation by opposing effects mediated by the μ-opioid and nociceptin/orphanin (nociceptin) FQ (NOP) receptors [22]. Activation of the μ-opioid receptor by low doses of buprenorphine stimulates cell maturation while high buprenorphine levels induce an inhibitory effect on oligodendrocyte differentiation mediated by the NOP receptor.…”